The present invention relates generally to systems and techniques for reducing systematic offsets/errors within integrated circuits caused by inaccuracies in thin film resistors, and more particularly to structures and methods for accomplishing reduction of the systematic offsets/errors without use of laser trimming or other type of trimming (such as electronic blowing of fuses) by using “layout grid address unit” (AU) reticle changes in an integrated circuit mask set.
High precision integrated circuits, such as R-2R DACs (digital to analog converters) and string DACs, usually rely on closely matched thin film resistors to achieve their high precision. The degree of achievable matching of thin film resistors is limited by the capability of the manufacturing process and also by the integrated circuit layout topology. Presently, most thin film resistors used in integrated circuit DACs, ADCs and the like are composed of nichrome, sichrome, or various common alloys thereof such as carbon-doped sichrome, aluminum nichrome, and tantalum nitride. In the prior art, a single “linear via” is provided at each end of a thin film resistor for making electrical contact to that end. In order to make a minimum resistance change or adjustment to the value of the thin film resistor by making a mask change, the effective length of the resistor is changed by changing the location of the linear via. The amount of change in the location of the via is an amount equal to an integral number of minimum layout grid address unit increments of the particular layout grid being utilized.
The resistances of thin film resistors can be adjusted or “trimmed” in various ways. Laser trimming of thin film resistors before packaging is a common but very expensive option. For example, the cost of an un-trimmed integrated circuit chip might be only approximately 10 cents, but a single laser trimming of its thin film resistors might cost approximately an additional 60 cents. Furthermore, in some cases, laser trimming of the resistors might be required at multiple temperatures, costing an additional 60 cents for trimming at each required temperature. The final cost of a packaged, laser-trimmed integrated circuit typically is at least three times greater than the cost if laser trimming is not required.
At the present state-of-the-art, the most accuracy that has been achieved in the prior art for low cost untrimmed, packaged DACs and ADCs has been approximately 12 bit resolution/accuracy.
The making of highly precise adjustments to the resistance of thin film resistors by modifying reticles of the integrated circuit mask set so as to modify the location of one or both of the linear vias at the ends of the resistors requires the ability to make precise measurements of the appropriate mask set reticles. The dimensions of mask set reticle features typically are magnified by 5× relative to the corresponding images optically projected onto the chip. This diminishes the size of any actual defect on the reticle by a factor of 5 by optically reducing the reticle image to the corresponding image on the chip during photolithography. Therefore, precise measurements of the 5× reticle, rather than of the actual thin film resistors on the chip, must be made in order to make precise mask set adjustments that will result in the desired modification of the resistance of a thin film resistor. Unfortunately, the large size of the corresponding features of a 5× mask set reticle is beyond the capabilities of the measurement instruments utilized in the making the reticles. Consequently, it would be very expensive and impractical to make precise adjustments by making mask set changes to minimize or eliminate systematic errors caused by mismatching of thin film resistors.
Thus, there is an unmet need for an improved thin film structure and technique which avoids the need to obtain precise measurements of mask set reticle features (the large size of which precludes use of available measurement instruments) in order to determine the magnitude of reticle changes needed to achieve desired values of, or ratios of, resistances of thin film resistors.
There also is an unmet need for an integrated circuit resistor structure and method which allows achievement of high accuracy thin film resistor ratios without the high expense of laser trimming.
There also is an unmet need for an integrated circuit resistor structure and method which allows achievement of precise thin film resistor ratios without laser or other kinds of trimming and with only a single modification of a single integrated circuit mask reticle.
There also is an unmet need for a way of substantially increasing the resolution/accuracy of untrimmed integrated circuit DACs and ADCs and the like.